In time synchronized Frequency Hopping Spread Spectrum (FHSS) networks, a PN (Pseudo random Number) code that determines the hopping sequence must be time-synchronized between nodes of the network. When a node has lost synchronization, it must enter an acquisition mode to regain synchronization.
In general, one central device (herein called a Coordinator Device 130 or CD) is used to which all other devices (herein called End Devices 120 or ED) are synchronized. A network can be built using multiple CDs 130 and EDs 120. To minimize collisions, each CD 130 can have its own sequence of frequency hops here called hopping domain.
One known method is to use a sliding correlator, as discussed by Prakis, “Digital Communications,” Fourth Edition, and Sklar, “Digital Communications”, Second Edition. This correlator performs a serial search that is generally time consuming. It means that the searching End Device (ED) 120 needs to operate its receiver for a long time consuming battery power. Another method, discussed in Chipcon Application Note AN014, is more practical: When an ED 120 lost synchronization it might search all the channels to try to find the Beacon packet of the CD 130. The Beacon packet is send e.g. for 4 ms every 64 ms meaning that the CD can be silent 60 ms every 64 ms. Also in FHSS the Beacon is sent on a random frequency. All this makes the searching process time consuming and may also reduce the battery life of the ED 120.